Urea-formaldehyde (UF) resins and other formaldehyde-condensed resins, which are toxic petroleum-based adhesives, have been used as wood adhesives for many years. However, the level of formaldehyde gas emission from UF-bonded products is heavily regulated by law, particularly with respect to indoor use. In addition, the high cost of most formaldehyde-condensed resins and the poor moisture resistance of UF resins have resulted in the need for alternative wood adhesives.
There is a growing interest to develop wood adhesives from renewable substances to reduce the dependency on petroleum-based chemicals and to expand the non-food use of agricultural commodities. In particular, uncertainty in future supplies of petroleum-derived chemicals and stringent regulations on toxic emissions from building materials bonded with certain synthetic resins have compelled the forest products industry to reevaluate wood adhesives from renewable substances. The agriculture industry also is eager to invest in researching nonfood industrial uses of agricultural products to expand their markets. A significant challenge is to develop resin adhesives from renewable substances meeting stringent performance requirements at reasonable costs.
As an alternative to petroleum-based adhesives, protein glues, such as casein, blood, and soy glues, reached their peak use in the 1960s and faded out in the 1970s. Most of the important research on formulating wood adhesives with protein occurred before 1960.
U.S. Pat. No. 2,817,639 to Ash et al. disclosed a plywood glue containing high blood solids blended with a commercial phenol-formaldehyde (PF) resin ail sodium silicate. This required a specific mixing procedure to avoid a high viscosity problem. Lambuth, "Blood Glues," in Handbook of Adhesives, I. Skeist, ed., Reinhols Publishing Corp., NY (1962) discloses the interactions between different types of PF resins and blood albumin. PF resins of simple structure behave much the same as aromatic alcohols, causing a change in blood glue consistency from gelatinous to granular, often accompanied by a reduction in viscosity. Alkaline phenolic resins of moderate complexity do not disperse blood proteins to a great extent. Highly complex and reactive PF resins, however, cause gelation of blood proteins, creating a major viscosity problem at any mixing combination. U.S. Pat. No. 2,368,466 to Golick et al. disclosed an exterior phenolic plywood glue containing up to 70% dried blood. This glue was prepared by first reacting cresol and casein with formaldehyde at room temperature to form an emulsion. This reaction was catalyzed by NH.sub.4 OH. Subsequently, this emulsion was mixed with blood albumin at room temperature to form a smooth glue. The casein did not react with PF resins, and the fluid glue contained finely precipitated particles of blood albumin-phenol-aldehyde complex.
U.S. Pat. No. 3,153,597 to Weakley et al. disclosed moisture resistant plywood adhesives by cross-linking casein with dialdehyde starch.
U.S. Pat. No. 5,593,625 to Riebel et al. describes methods of preparing a legume-based thermosetting resin for producing rigid biocomposite materials. This resin was formulated by cross-linking soy flour with methyl diphenyl isocyanate and mixing the resin with paper fibers in a ratio about 4:6. A molded composite product was produced.
U.S. Pat. No. 5,371,194 to Ferretti describes a method of synthesizing a thermosetting resin from a mixture of protein and carbohydrate by ammoniation. This ammoniated resin was developed as a moisture resistant binder for pelletizing coal fines.
Further, the use of soy protein isolates and phenol-resorcinol-formaldehyde (PRF) in combination for lumber finger jointing is currently under mill trial studies. In this finger jointing system, the soy protein isolates and PRF are separately applied onto different fingers, and upon joining these fingers, interactions between soy protein and PRF first immediately cause the adhesive to gel and, eventually, the adhesive is completely cured in several hours.
Although great effort has been undertaken to develop protein glues, such glues were slow curing and had poor moisture resistance. As a result, the art sought to develop synthetic resin adhesives. Detlefsen, "Blood and Casein Adhesives For Bonding Wood," in Adhesives from Renewable Resources, Hemingway et al., eds., ACS Symposium Series 385, Am. Chem. Soc., Washington, D.C. (1989).
The present invention is directed to overcoming the deficiencies in protein glues and their use in manufacturing composite products.